The invention is directed to a new process and a new composition of matter. It deals with the formation of selenide, sulfide, and mixed selenide-sulfide on metal or metal coated substrates requiring temperatures in excess of 200° C. Specifically, it solves the problem of crack formation which commonly occurs when copper, indium, gallium, diselenide, i.e., CuInGaSe2 (CIGS) is deposited onto a molybdenum coated substrate. It also improves the adhesion of the CIGS film to the Mo layer. In the past, both of these issues have impeded the development of CIGS based photovoltaic (PV) devices on flexible polymer substrates.
In a more general way, the invention also applies to other substrates as well, such as Mo coated glass and Mo coated metal foils, as well as glass, polymer and metal foil substrates coated with niobium (Mb), tantalum (Ta), tungsten (W), titanium (Ti), for example. The invention further applies to methods of forming CIGS films by selenization of precursors films. Such precursor films can include metals or compounds in the form of uniform layers or powders as long as the selenide, sulfide or mixed selenide-sulfide is formed on metal film such as, for example, Mo, Ta, W, and TI. In cases where selenide or mixed selenide-sulfide films are formed directly on metal foils, the invention improves the adhesion of the film to the substrate.
An example of the cracking found in prior art composite films can be seen in the Scanning Electron Microscopy (SEM) micrograph shows in FIG. 1 where the CIGS layer (white area) was deposited on a Mo film (dark area) which coated a polyimide film. Such cracks dramatically reduce the performance of CIGS based PV devices. This consequently is illustrated in FIGS. 2a and 2b. FIG. 2a is the photograph of a sample containing 4 devices. On the same there are two orthogonal contact lines (labeled 1 and 2 in FIG. 2.2a) to the underlying Mo film that is the electrical back contact to the devices. FIG. 2b gives the current density vs voltage (JV) characteristics of one of the devices utilizing one and both of the contact lines. When only the contact line 1 is used, current collection from the device is severely limited compared to the case when contact line 2 is placed orthogonal to line 1. The explanation for this is that, in this particular case, the cracks are mostly parallel to contact line 1 and therefore current can't be collected by the contact line 1 alone.
U.S. Pat. Nos. 6,310,281 B1 and 6,372,538 B1 dated respectively Oct. 30, 2001 and Apr. 16, 2002 allege that during the fabrication of CIGS photovoltaic modules on polyimide substrates, cracking of sputter deposited Mo layer during subsequent downstream processing can be avoided by the addition of oxygen or water vapor into the sputtering gas. These disclosures allege that as a result of this addition of oxygen or water vapor into the sputtering gas, oxygen is entrained into the Mo layer creating a higher level of internal compressive stress in the Mo layer as a result of which “these layers are able to lerate temperature changes that occur in subsequent processing without suffering temperature-induced cracking and fracturing.” However, these patents fail to state the amount of oxygen entrained into the films, and instead, give the range of the relative amount of water vapor or oxygen in the sputtering gas. The amount of oxygen in the Mo is intimately related to the design and operation of the sputtering system used for the deposition of Mo layers. These patents do not suggest an amount of oxygen concentration in the Mo film which is necessary to suppress the cracking of such films during the fabrication of CIGS photovoltaic modules. Indeed, studies to the issuance of these patents have concluded without exception, that Mo cracking is an unresolved problem.